A key to a well-managed, educational classroom: making sure every student has something constructive to do at every moment. Sounds like a no-brainer, and yet a lot of times students are off task because they have no task. This especially happens when students finish an assignment while other students are still working on it. That’s why you need to include an “early finisher” activity with every assignment. And be sure to put your expectations in writing on the assignment sheet, board, or projector screen.
As for possible activities, options include extension activities related to the current topic, brain teasers, journal writing, silent reading, and educational games (one of my students’ favorites was the math game “24”—I kept cards in my tool belt, and handed them out as students finished an assignment). Whatever you choose, just be sure it’s constructive—no inane busy work (down with “word searches,” especially for older kids).